Expensive energy, in phase (5)
I believe the answer is:
steep
'expensive' is the definition.
(I've seen this before)
'energy in phase' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mct).
'in' is an insertion indicator.
'phase' becomes 'step' (I can't explain this - if you can you should believe this answer much more).
'e' placed into 'step' is 'STEEP'.
Can you help me to learn more?
(Other definitions for steep that I've seen before include "Stiff - thoroughly wet" , "Vertiginous; excessive" , "Soak thoroughly - precipitous" , "not reasonable" , "Unreasonably dear - soak" .)